Molecular sieves of the crystalline aluminosilicate zeolite type are well known in the art, and now comprise over 150 species of both naturally occurring and synthetic zeolites. In general, the crystalline zeolites are formed from corner-sharing AlO2 and SiO2 tetrahedra and are characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent crystal structure.
Other crystalline microporous compositions are known which are not zeolitic but which exhibit the ion exchange and/or adsorption characteristics of the zeolites. These include: 1) a pure silica polymorph, silicalite, having a neutral framework containing neither cations nor cation sites as disclosed in U.S. Pat. No. 4,061,724; 2) crystalline aluminophosphate compositions disclosed in U.S. Pat. No. 4,310,440; 3) silicon substituted aluminophosphates as disclosed in U.S. Pat. Nos. 4,440,871 and 4) titanium substituted aluminophosphates as disclosed in U.S. Pat. No. 4,500,651.
Molecular sieves are usually hydrothermally synthesized from a reaction mixture in a batch reactor. In this type of process, all ingredients are added to a reactor thereby forming a gel. The gel is next stirred and heated for a sufficient time to crystallize the zeolite. The drawbacks to a conventional process include limitations on the control of size and morphology of crystals, limitations on the solids content, generation of waste products which can't be recycled and large capital investments. Accordingly, the industry is continuously conducting research to improve the manufacture of molecular sieves.
For example, U.S. Pat. No. 4,314,979 discloses a continuous process for preparing zeolite A. The process involves mixing solutions containing aluminum and silicon and flowing the mixture to a crystallization reactor to crystallize zeolite A. U.S. Pat. No. 5,389,358 discloses a process for synthesizing zeolites by first nucleating crystals and then adding solutions which contain the reactants followed by aging in order to crystallize the zeolite. Finally, U.S. Pat. No. 3,425,800 describes a continuous process for synthesizing zeolite A or X in which aqueous solutions of the reactants are mixed to form a gel, the gel is heated and then supplied to a stratified crystallization zone where the crystals form.
C. S. Cundy et al., in Zeolites, Vol. 15, 353–372 (1995), discloses a process for synthesizing the zeolite ZSM-5. The process involves filling a reactor with a slurry of seed crystals in a suitable liquid. To this mixture there are continuously added sources of aluminum and silicon with intermittent removal of product such that the reactor is filled to a constant level. In a second article by the same authors Zeolites, Vol. 15, 400–407 (1995), it is disclosed that when aluminum and silicon are added at a faster rate than crystal growth, a high nucleation rate is observed.
Finally, U.S. Pat. No. 6,773,694 B1 discloses a process for synthesizing molecular sieves by adding to a slurry of seed crystals nutrients which are sources of the framework elements, e.g. Al, Si of the molecular sieve. Nutrients are added at a rate which essentially equals the crystal growth rate such that no gel is formed and there is no nucleation of new crystals.